The present invention relates to an electrochromic element.
Electrochromic (EC) elements are at present under discussion for varied applications, for example, for vehicle and building glazings, for mirrors with variable reflectance, in the display technology, etc. In this context, the optical properties of a layer system such as absorptivity, transmissivity and reflectivity are controlled via electrical signals.
A conventional EC system, for example, according to German Patent Applications DE 44 09 470 A1 or DE 36 43 692 A1, is typically composed of a multilayer structure including an electrochromic functional layer, an ion-conducting electrolyte with dissolved salt, and a storage layer which is capable of absorbing or giving off mobile ions. The ion transfer is controlled via large-surface electrodes, at least one electrode being transparent.
A characteristic element of the known systems are special inorganic or organic functional layers which feature a reversible coloring by changing the oxidation state. Examples of electrochromic materials of that kind are the oxides of the transitional metals such as tungsten, niobium, vanadium, titanium, tantalum, nickel, and others, as well as polyaniline, polythiophene, and others. The electrochrornic mechanism of these substances is based on intercalation of mobile ions on interlattice positions of the EC material, resulting in discrete color centers having a material-specific absorption band. To make it easier for the ions to penetrate into the EC material, usually small ions such as lithium or hydrogen (protons) are used.
Moreover, it is a common feature of the known EC materials that they possess both electronic and ionic conductivity. Besides the change in color, this exceptional feature is an indispensable prerequisite for the EC function in the known layer configurations (double charge injection).
The fundamental functional mechanism of the conventional elcctrochromism, the intercalation of mobile ions, limits the performance of such systems with respect to coloring depth, switching speed, and stability. Thus, only a limited number of materials exhibits a technically usable electrochromic effect involving a sufficient coloring in the visible or infrared wavelength ranges. Moreover, a color-neutral coloring can be produced only with limitations, for example, at the cost of the transmission range or the coloring efficiency. High switching speeds, as are required in safety-relevant fields (for example, the windshield of vehicles), are generally not attained due to the ion transfer in the electrochromic layer, in the electrolyte, and in the storage layer, and because of the transfer resistances and adhesion potentials to be overcome in the process.
The most frequently used electrochromic materials such as tungsten oxide or polyaniline are, moreover, difficult to handle in practical use so that the production of stable, reproducible EC elements of high optical quality requires considerable technical outlay and has not been satisfactorily achieved so far.
In document WO 97/23578 and the corresponding U.S. Pat. No. 5,876,633, small highly doped separate particles made of SnO2 are described which permit a more intense electrochromic coloring than compact thin films of SnO2 or solid bodies. In small SnO2 particles of that kind, it is moreover possible to achieve considerably higher doping concentrations (Sb, Nb) so that the charge carrier density is sufficient to partially absorb visible light. Because of this, these particles possess an intrinsic natural color. In document WO 97/23578, a range of 50 nm to 20 micrometers is specified for the particle size. The particles are used, in particular, in electrochromic displays.
Document WO 98/35267 describes an electrochromic display in which a nanoporous semi-conductive metal oxide is used as a carrier for a redox chromophore material whose electrochromic properties are controlled.
U.S. Pat. No. 5,724,187 discloses an electrochromic device whose functional element is composed of an electrochromic film which is optionally doped, for example, for shifting the spectral absorption edge of the base material.
The object of the present invention is to devise an electrochromic element by means of which considerably shorter switching times and a higher optical switching range (modulation depth) are achieved combined with good color neutrality. Besides, the intention is for the electrochromic element to be manufacturable cost-effectively and in a manner that it has a large surface and a high long-term stability.
According to the present invention, the electrochromic functional layer of the element is a transparent layer which is made of a doped semiconductor and structured in the enanometer range. The layer features structure size in the range smaller than 50 nm. In this context, it is not required for the semiconductor material to be electrochromic in the conventional sense so that very stable base substances can be used.
Suitable for applications in the visible optical spectral range are, for example, tin oxide SnO2, zinc oxide ZnO, cadmium oxide CdO, titanium oxide TiO2, tantalum oxide Ta2O5 while silicon Si, germanium Ge, zinc selenide ZnSe and others are especially suitable for infrared applications.
By doping such semiconductor materials with foreign atoms, a technologically simple way ensues for the absorptivity of the nanoporous EC layers to be individually adjusted in wide ranges of the visible and infrared wavelength ranges. The operating range of the element according to the present invention can moreover be adjusted to a specific spectral range by adjusting the structure sizes of the electrochromic functional layer.
The regions of the nanoporous functional layer are connected, forming an electrically conductive network which has a large inner surface and which, on one hand, is connected to the electrode in a well-conducting manner and, on the other hand, is in direct contact with the ion-conducting electrolyte. Ideally, the pores are completely filled with electrolytic material.
The change of the optical properties of such a doped nanoporous semiconductor functional layer is effected by applying an electrical field as a result of which the absorptivity of the EC material is either increased or reduced, depending on the polarity. The energetic position of the maximum absorption range or transmission range lies within the visible or in the infrared wavelength range, depending on the degree of doping and the structure sizes, and can amount to up to 75%.
Unlike conventional EC elements, in which discrete color centers are produced in the electrochromic material by intercalation of small ions (hydrogen or lithium), in the case of the EC element according to the present invention, a change of the optical properties of is achieved only by accumulation of mobile ions on the boundary surface of the functional layer. Thus, an electrochromic effect is also achieved with large ions.
The use of doped tin oxide SnO2 as the material for the electrochromic functional layer is of outstanding significance since the tin oxide constitutes the preferred material for transparent electrodes due to its chemical stability and high electric conductivity. Thus, it is possible to prepare the EC functional layer and the electrode from the same base material. The electrode itself, however, needs not to have the nanostructuring according to the present invention.
The electrochromic functional layer can be manufactured in a particularly advantageous manner from a colloidal solution of dispersed nanoparticles which are composed of a doped semiconductor material and have a particle size smaller than 50 nm.
In this context, the functional layer can be obtained from the colloidal solution, in particular, by spin coating, by sol-gel dip coating, or by a spray pyrolysis method, including a subsequent heat treatment.
Alternatively, the functional layer can be manufactured using an electrochemical method such as porous etching or porous anodization of metals.
A further suitable method for manufacturing the functional layer is the coating with a nanostructured mask which is selectively removed later.
The advantage of the present invention lies above all in that the functional layers which are nanostructured according to the present invention make it possible to attain strong electrochromic effects such as a high transmission range. In this context, the switching times are considerably shorted than with the known electrochrornic elements. Moreover, the manufacture of electrochromic cells can be considerably simplified.